<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>超时与网络异常</title>
    <style>
        #result{
        width: 200px;
        height: 100px;
        border: solid 1px #90b;
    }
    </style>
</head>
<body>
    <button>点击发送请求</button>
    <div id="result"></div>
    <script>
        const btn = document.getElementsByTagName('button')[0]
        const result = document.querySelector('#result')

        btn.addEventListener('click',function(){
            const xhr = new XMLHttpRequest()
            // 超时设置  2s 设置
            xhr.timeout = 2000
            // 超时回调
            xhr.ontimeout = function(){
                alert('网络异常，请稍后重试')
            }
            // 网络异常回调
            xhr.onerror = function(){
                alert('你的网络似乎出了一些问题')
            }
            xhr.open("GET",'http://127.0.0.1:8000/delay')
            xhr.send()
            xhr.onreadystatechange = function(){
                if(xhr.readyState === 4){
                    if(xhr.status >= 200 && xhr.status <= 300){
                        result.innerHTML = xhr.response
                    }
                }
            }
        })
    </script>
</body>
</html>